[SPARK-17529][CORE] Implement BitSet.clearUntil and use it during merge joins

## What changes were proposed in this pull request?

Add a clearUntil() method on BitSet (adapted from the pre-existing setUntil() method).
Use this method to clear the subset of the BitSet which needs to be used during merge joins.

## How was this patch tested?

dev/run-tests, as well as performance tests on skewed data as described in jira.

I expect there to be a small local performance hit using BitSet.clearUntil rather than BitSet.clear for normally shaped (unskewed) joins (additional read on the last long).  This is expected to be de-minimis and was not specifically tested.

Author: David Navas <davidn@clearstorydata.com>

Closes #15084 from davidnavas/bitSet.